We are studying the molecular organization of the cell envelope of the leprosy bacillus (Mycobacterium leprae) by the analysis of high-resolution freeze-fracture images of the bacteria. The crystalline patterns found in these micrographs will be characterized by laser optical diffraction. Because M. leprae is noncultivable in vitro, liver fragments from infected armadillos and nude mice are used. We are comparing the structure of the envelope of the leprosy bacillus with that of other pathogenic mycobacteria: M. lepraemurium and M. avium. It is our aim to investigate the structural basis of mycobacterial escape from destruction inside the macrophage.